Environmentalists sued Wednesday to block proposed water transfers from Northern California to the drought-plagued south San Joaquin Valley, arguing that the plan fails to protect the fragile Delta.

Under the federal government's plan, willing sellers in the north would allow water to flow down the Sacramento River into the Delta, where the giant export pumps near Tracy would deliver it to southland farms, where very little water is available this summer.

But the Stockton-based California Sportfishing Protection Alliance warns that the extra pumping could draw tiny, threatened Delta smelt from the west Delta into the central Delta, where they could be harmed by warmer water temperatures or by the massive pumps themselves.

The alliance, along with another plaintiff environmental group, AquAlliance, also argues that groundwater in the north will be overdrawn in order to provide for arid lands in the south that should never have been planted in permanent orchards.

The lawsuit was filed at U.S. District Court in Sacramento. It asks a judge to stop the transfers or order more intensive environmental review. The U.S. Bureau of Reclamation is named as a defendant.

Bill Jennings, head of the sport-fishing alliance, said drought-related emergency actions to weaken water quality standards in the Delta this summer make the transfers even more dangerous. He said a new analysis shows the amount of water allowed to remain in the Delta and flow out to San Francisco Bay is even lower than reported.

"We ought not to be exporting water this year if we don't have Delta outflow," Jennings said.

The amount of water to be transferred south could be as much as 175,226 acre-feet, the lawsuit says. By way of comparison, that's more than half the capacity of New Hogan Lake, east of Stockton.

A bureau spokesman in Sacramento said he could not comment on pending litigation.

Mike Wade, head of the California Farm Water Coalition, said the lawsuit is "extremely harmful" to water users in the south.

"One region of the state has some supply they can share with another region and help balance some of the water supply challenges in a year like this," Wade said. "These transfers need to go through."

Contact reporter Alex Breitler at (209) 546-8295 or abreitler@recordnet.com. Follow him at recordnet.com/breitlerblog and on Twitter @alexbreitler.